How to submit a form to custom url with a different HTTP method?

Question

I'm using simple_form gem to build my forms, and I'd like to submit a form to "/messages/delete_all" with the DELETE method. How can I do that with simple_form?

Here is what I have so far:

=simple_form_for :messages, :url=>{:action=>"delete_all"} do |f|    
   =f.button :submit, "Delete"

This doesn't seem to work, though. It submits to "/messages/delete_all" with POST.